1. Field of the Invention
The present invention relates to an improved video/intercarrier sound detecting circuit in a television receiver. More specifically, the present invention relates to a video detecting circuit and an intercarrier sound signal generating circuit particularly suited for implementation in an integrated circuit in a television receiver.
2. Description of the Prior Art
In view of a decreased mixed modulation of synchronous detection of a video intermediate signal for obtaining a detected video signal, a synchronous detector has been proposed and widely used for detection of a video signal from a video intermediate frequency signal in television receivers. It has also been proposed and practiced that a common multiplier is utilized for synchronous detection of a video intermediate frequency signal for providing a detected video signal and for multiplication of a video intermediate frequency signal for providing an intercarrier sound signal in the form of a sound signal of the frequency 4.5 MHz which is a beat of the video carrier frequency 58.75 MHz and the sound carrier frequency 54.25 MHz.
FIG. 1 shows a block diagram of the above described typical conventional video synchronous detector which is commonly utilized for generation of an intercarrier sound signal. Referring to FIG. 1, the video intermediate frequency signal from the final stage of the video intermediate frequency amplifier is applied to a video carrier frequency signal extracting circuit 1 and a multiplier 2. The video intermediate frequency carrier signal of the frequency 58.75 MHz obtained from the video intermediate frequency carrier signal extracting circuit 1 is also applied to the other input of the multiplier 2. The multiplier is responsive to the video intermediate frequency carrier signal from the video intermediate frequency signal extracting circuit 1 and the video intermediate frequency signal from the video intermediate frequency amplifier to effect synchronous detection of the video intermediate frequency signal as a function of the video intermediate frequency carrier signal to provide a detected video signal, as well known to those skilled in the art. The detected video output is applied to a video circuit 3. As well known to those skilled in the art, the multiplier 2 also provides an intercarrier sound signal of the frequency 4.5 MHz that is a beat of the video carrier frequency 58.75 MHz and the sound carrier frequency 54.25 MHz (Japanese Television Standard). The intercarrier sound signal is applied to a sound circuit 4.
FIG. 2 shows a schematic diagram of the video intermediate frequency carrier signal extracting circuit 1 and the multiplier 2, as implemented in an integrated circuit. The video intermediate frequency signal as received as a differential signal at input terminals 5 and 6 is applied to a pair of emitter followers Q1 and Q2 and the output video intermediate frequency signals from the emitter followers Q1 and Q2 are applied to the base electrodes of a pair of differential operating transistors Q3 and Q4, respectively, constituting a differential amplifier that constitutes the video intermediate frequency carrier signal extracting circuit 1. The collector electrodes of the differential operating transistors Q3 and Q4 are coupled to a parallel connection of an inductance coil L1 and a capacitor C1 provided externally of the integrated circuit that constitutes a tuning circuit 7 tuned to the central frequency of 58.75 MHz. Therefore, the video intermediate frequency carrier signal is extracted at the collector electrodes of the above described differential operating transistors Q3 and Q4. The collector electrodes of the differential operating transistors Q3 and Q4 are coupled to each other through diodes D1 and D2 in opposite polarities. Therefore, the video intermediate frequency carrier signal as obtained at the collector electrodes of the differential operating transistors Q3 and Q4 are subjected to a limiting function by these diodes D1 and D2, whereby the video intermediate frequency carrier signal is pulsed or is converted into a pulse form. The pulse signal of the video intermediate carrier frequency is obtained at each of the collector electrodes of the differential operating transistors Q3 and Q4 in an opposite polarity to each other and is applied to the corresponding one of a pair of emitter followers Q5 and Q6. The pulse outputs from the emitter followers Q5 and Q6 are applied as a switching control signal to the base electrodes of a pair of upper differential operating transistors Q7 and Q8 and another pair of upper differential operating transistors Q9 and Q10 of a double balanced synchronous detector that constitutes the multiplier 2. On the other hand, the base electrodes of a further pair of lower differential operating transistors Q11 and Q12 of the above described multiplier 2 are connected to receive the video intermediate frequency signal obtainable from the emitter followers Q1 and Q2. As a result, multiplication is made of the video intermediate frequency signal from the emitter followers Q1 and Q2 and the above described pulse control signal as applied to the upper differential operating transistors. As a result, a detected video signal is obtained through synchronous detection and an intercarrier sound signal of the frequency 4.5 MHz is obtained as a beat of the video and sound carrier frequencies at output terminals 8 and 9 as a differential output form. The video signal and the intercarrier sound signal thus obtained are separately extracted by individual filters, not shown, to be provided in the video and sound circuits, respectively.
Since in the above discussed conventional video/intercarrier sound dectecting circuit a detected video signal and an intercarrier sound signal are obtained by the use of a single common multiplier structured to make multiplication of the video intermediate frequency signal by a pulse output of the video intermediate frequency carrier signal as a switching operation, buzz noises and the like are liable to occur for the reasons to be described in the following, that degrade the sound quality in television receivers. More specifically, since the video intermediate frequency signal is obtained from a televised signal, the amplitude of the extracted video intermediate frequency carrier signal is liable to fluctuate, as shown in FIG. 3, in accordance with amplitude modulation. Therefore, when the extracted video intermediate frequency carrier signal is pulsed by means of the limiting diodes D1 and D2 to unify the signal level, a portion a having a large amplitude is completely pulsed as shown as a' in FIG. 4, while portions b and c having smaller amplitude is incompletely pulsed as shown as b' in FIG. 4 or is not pulsed at all as shown as c' in FIG. 4, with the result that pulsing is diversified depending on the amplitude modulation, and thus a residual amplitude component contained in the switching control signal as applied to the multiplier 2.
Another problem is caused with the above described conventional video/intercarrier sound detecting circuit as shown in FIG. 2. More specifically, when one of the limiting diodes D1 and D2 becomes conductive, the other is cut off to function as a capacitance. As a result, it follows that these diodes D1 and D2 exhibits a somewhat rectifying function with respect to the extracted video intermediate frequency signal, which causes harmonics with respect to the above described amplitude modulation component. This means that when a video signal of the frequency 2.25 MHz is received, a frequency component 4.5 MHz that is the second harmonic of the above described video signal is applied to the base electrodes of the upper stage pairs of differential operating transistors Q7 and Q8 and Q9 and Q10 constituting the multiplier 2. As a result, a pseudo sound signal is obtained and is applied to the sound circuit 4, thereby to degrade the quality of sound.
In this context, a video/intercarrier sound detecting circuit of interest is described in the article, entitled "A New TV Video/Intercarrier Sound Detector IC" by Milton E. Wilcox, in IEEE Transaction on Broadcast and TV Receivers. The above discussed problems encountered in the video/intercarrier sound detector shown in FIGS. 1 and 2 are solved to some extent by the video/intercarrier sound detector described in the above referenced article. More specifically, in accordance with the above referenced article, a separate sound detector is employed. The approach used therein is a half wave equivalent of the multiplier which switches the video carrier with the sound carrier input signal to produce a desired difference frequency. The sound detecting circuit shown has a separate input for the sound carrier and applies the video carrier signal derived in the video detector to the base electrodes of the lower differential operating transistors of the half wave equivalent of the multiplier. The output is tuned to the sound intermediate frequency of 4.5 MHz. Nevertheless, the problems discussed previously in conjunction with the sound quality are not fully solved by the video/intercarrier sound detector in the above referenced article. More specifically, in accordance with the video/intercarrier sound detector in the above referenced article, the video carrier signal derived in the video detector is applied to the sound detector, which means that the video carrier signal of a very large signal level is applied to the sound detector rather in the form of a switching control signal. Thus, according to the above referenced article, the sound detector does not operate in a linear manner but rather operates in a non-linear manner. The fact that the sound detector operates rather in a non-linear manner is liable to cause harmonics, which degrades the quality of sound, as fully discussed previously. In this context, there is room for improvement in the video/intercarrier sound detector described in the above referenced article.